1. Field of the Invention
The present invention relates to a cathode adapted for use in a liquid fuel type polymer electrolyte fuel cell, a membrane electrode assembly using the cathode, and a liquid fuel cell using the cathode.
2. Description of the Related Art
Fuel cells have attracted attention as a source of clean electric energy. A direct methanol fuel cell (DMFC) using a polymer electrolyte membrane permits miniaturization of the fuel cell, compared with other fuel cells. As a result, extensive research is being conducted on the direct methanol fuel cell as a power source for portable equipment such as a notebook-type personal computer or a cellular telephone.
The membrane electrode assembly of the DMFC using the polymer electrolyte membrane is a laminated structure constructed such that an anode diffusion layer, i.e., a so-called “current collector”, an anode catalyst layer, i.e., a so-called “fuel electrode”, a proton conductive membrane, a cathode catalyst layer, i.e., a so-called “oxidizing agent electrode” and a cathode diffusion layer, i.e., a so-called “current collector”, are laminated one upon the other in the order mentioned. The catalyst layer is a mixed body comprising a catalytic activity substance, an electrically conductive substance, a proton conductive substance, and pores. In the case of a supported catalyst comprising an electrically conductive substance used as a carrier, the catalyst layer is in many cases a mixed body comprising the supported catalyst, the proton conductive substance, and pores.
If a mixed fuel having methanol and water is supplied into the anode catalyst layer, with the air (oxygen) being supplied to the cathode catalyst layer, catalytic reactions denoted by chemical formulas (1) and (2) given below are generated in the fuel electrode and the oxidizing agent electrode respectively:
Fuel Electrode:CH3OH+2H2O→CO2+6H++6e−  (1)
Oxidizing Agent Electrode:6H++(3/2)O2+6e−→3H2O  (2)
The protons generated in the fuel electrode migrate to the proton conductive membrane. On the other hand, the electrons migrate to the cathode diffusion layer. The electrons supplied from the cathode diffusion layer are allowed to react with the protons supplied from the proton conductive membrane and with oxygen, with the result that a current flows between a pair of current collectors. In order to obtain excellent fuel cell performance, it is desirable for the fuel electrode and the oxidizing agent electrode to satisfy the four requirements given below:
(A) An appropriate amount of fuel should be supplied to the fuel electrode.
(B) The catalytic reaction take place vigorously and promptly in the fuel electrode such that the reaction is carried out at triple phase boundaries of gas-catalyst-electrolyte.
(C) The electrons and the protons should migrate smoothly in both the fuel electrode and the oxidizing agent electrode.
(D) The reaction product should be discharged promptly from both the fuel electrode and the oxidizing agent electrode.
If the cathode is constructed to form an electrode structure that permits obtaining excellent fuel cell performance at low air feeding rates (low air performances), it is possible to miniaturize the DMFC. Therefore, it is desirable for the cathode to have a construction that permits promoting the air diffusion. The cathode that is most widely used nowadays is prepared by forming a slurry mixture such as a mixture containing granular catalysts and proton conductors together with a solvent on a carbon paper (cathode diffusion layer) or a proton conductive membrane by, for example, the coating method, a transfer method, or a spray method. However, the known cathode is incapable of obtaining sufficient fuel cell performance with a small air supply amount.
The cathode is very important in a polymer electrolyte membrane fuel cell (PEMFC) using a gaseous fuel such as a hydrogen gas. Such being the situation, extensive research is being conducted in an attempt to optimize the cathode structure. It is described in, for example, Japanese Patent Disclosure (Kokai) No. 2001-126738 that the electrode pore structure is optimized by changing the carrier. In the case of this patent document, it is proposed to introduce a fibrous carrier into the catalyst layer so as to optimize the electrode pore structure. It is also known in the art to suppress water flooding by the formation of an intermediate layer. However, compared with the study for the PEMFC, study of the optimization of the cathode of the DMFC is insufficient. In the DMFC, the so-called “cross-over” phenomenon is generated wherein the liquid fuel, e.g., a methanol aqueous solution, migrates through the electrolyte membrane to reach the cathode, though the cross-over phenomenon is quite small in the PEMFC using a hydrogen gas as a fuel. The cross-over phenomenon has a detrimental effect on the electrode catalytic reaction performed by the cathode. Also, it is more difficult to diffuse oxygen into the cathode of the DMFC than to diffuse oxygen into the cathode of the PEMFC. Various measures such as the optimization of the porosity and the pore diameter have been explored in order to optimize the cathode of the DMFC. For example, it is disclosed in Japanese Patent Disclosure No. 2003-317742 that carbon nanotubes are used for preparing a catalyst layer having high porosity. Also, it is disclosed in Japanese Patent Disclosure No. 2003-200052 that, in order to optimize the pore structure, fibers differing from each other in diameter distribution are mixed so as to form two different kinds of pore distribution, where the fine fibers are supported with the catalyst particles.
However, none of the proposals in the patent documents quoted above provides a sufficient measure. There seems to be room for further improvement in respect of the low air performance.